How do I add a composite effect to a scene using material index?
It is unclear to me what you mean by shockwave, but here's what I think.

For better control separate the different elements in different layers.



Then set different render layers, so that the particle layer excludes the torus and the layer for the torus excludes the particles.



Then mix them in the compositor using alpha over, you won't need the material index:


click on the image to enlarge